New checks/update getter for molar_mass in constituents object

[peverwhee]

Description

It would be nice to have:

1. A check of whether molar_mass is set to 0 (which would return an error message) since molar_mass is often used as a denominator in unit conversions
2. Return an errmsg from the getter if the molar_mass = kphys_unassigned (probably no error code, but just would be nice to have a clean way for the host to know if molar mass has been assigned)

Solution

1. Add check to instantiate() that returns an errmsg (and probably and errcode) if molar mass is being set to 0
2. Modify ccp_molar_mass to return a message when

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Related to (optional)

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